U.S. patent application number 13/255777 was filed with the patent office on 2012-01-05 for shaped cellulose manufacturing process combined with a pulp mill recovery system.
This patent application is currently assigned to KIRAM AB. Invention is credited to Lars Stigsson.
Application Number | 20120000621 13/255777 |
Document ID | / |
Family ID | 42728568 |
Filed Date | 2012-01-05 |
United States Patent
Application |
20120000621 |
Kind Code |
A1 |
Stigsson; Lars |
January 5, 2012 |
SHAPED CELLULOSE MANUFACTURING PROCESS COMBINED WITH A PULP MILL
RECOVERY SYSTEM
Abstract
Provided is a process for manufacturing shaped cellulose
materials from lignocellulose where a dissolving grade pulp is
manufactured and dissolved in an aqueous alkaline or acidic solvent
system forming a solution suitable for shaping new cellulose
structures including fibers, films and cellulose derivatives. At
least a part of the spent cellulose dissolving or cellulose shaping
chemicals are recovered in one or more unit operations in a pulp
mill chemical recovery cycle.
Inventors: |
Stigsson; Lars; (Bjarred,
SE) |
Assignee: |
KIRAM AB
BJA
SE
|
Family ID: |
42728568 |
Appl. No.: |
13/255777 |
Filed: |
March 8, 2010 |
PCT Filed: |
March 8, 2010 |
PCT NO: |
PCT/SE10/50256 |
371 Date: |
September 23, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61202517 |
Mar 9, 2009 |
|
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|
61272080 |
Aug 14, 2009 |
|
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Current U.S.
Class: |
162/158 ;
162/189 |
Current CPC
Class: |
C08B 16/00 20130101;
C08H 8/00 20130101; Y02P 20/10 20151101; D21C 9/10 20130101; D21C
3/26 20130101; C08B 37/0057 20130101; D21C 3/02 20130101; Y02P
20/127 20151101; D21C 11/00 20130101 |
Class at
Publication: |
162/158 ;
162/189 |
International
Class: |
D21H 17/00 20060101
D21H017/00; D21F 1/66 20060101 D21F001/66 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 11, 2009 |
SE |
0901175-0 |
Dec 28, 2009 |
SE |
0901615-5 |
Claims
1. A process for manufacturing shaped cellulose material from
lignocellulose comprising the steps of: a) providing a feed of
comminuted lignocellulosic material comprising cellulose,
hemicelluloses and lignin; b) separating lignin from
lignocellulosic feed material by cooking the material at a
temperature between about 110 to 200.degree. C. for a time period
of from about 1 hour to 6 hours in an aqueous solution comprising
soluble alkali, alkali earth metal or phosphorous compounds,
thereby forming a first stream of solid material enriched in
cellulose and a second stream rich in dissolved lignin; c) treating
the first stream of cellulose from step b) by at least one of
oxygen delignification, bleaching, and alkali extraction, to form a
cellulose pulp with a lignin content below about 2%. d) treating
spent liquor streams comprising dissolved lignin, alkali, alkali
earth, phosphorous or sulfur compounds, in a chemicals recovery
system comprising a spent liquor concentration unit, optionally a
causticising unit, and a gas generator or recovery boiler unit,
wherein one or more of fresh alkali, alkali earth metal, and
phosphorous or sulfur compounds, are recovered and reformed; e)
dissolving cellulose having a lignin content below about 2 wt % in
a solution comprising one or more of alkali, alkali earth, and
phosphorous or sulfur compounds, or dissolving cellulose in a
molten salt, the solution optionally comprising one or more
additives, thereby forming a substantially homogeneous solution
comprising dissolved cellulose, and thereafter shaping dissolved
cellulose into fibers, films or cellulose derivatives; f) directly
or indirectly recycling spent cellulose dissolving or cellulose
shaping chemicals comprising one or more of alkali, alkali earth
metal, and phosphorous and sulfur compounds, to one or more of the
chemical recovery units of step d) the spent chemicals discharged
from at least one of: f1) a cellulose dissolving step, f2) a
cellulose shaping or cellulose coagulation step, and f3) a shaped
cellulose washing step: and g) charging fresh cellulose dissolving
or cellulose shaping chemicals comprising one or more of alkali,
alkali earth metal, and phosphorous or sulfur compounds reclaimed
directly or indirectly from a chemicals recovery unit in step d) to
a cellulose dissolving or cellulose shaping step in e).
2. A process for the manufacturing shaped cellulose material from
cellulose pulp and the recovery and recycling of spent cellulose
dissolving and cellulose shaping chemicals, comprising the steps
of: h) providing a feed of cellulose pulp having a lignin content
below about 2%; i) dissolving cellulose from h) in a solution
comprising one or more of alkali metal, alkali earth metal, and
phosphorous or sulfur compounds, or dissolving cellulose in a
molten salt, the solution optionally comprising one or more
additives, thereby forming a substantially homogeneous solution
comprising dissolved cellulose; j) shaping the dissolved cellulose
from step i) into fibers, films or cellulose derivatives; and k)
recovering dissolving or cellulose shaping chemicals used in step
j) in one or more chemical recovery units selected from the group
consisting of a liquor concentration unit, a recovery boiler unit,
a gas generator unit, and optionally a causticising unit, the
chemicals recovery units integrated in a kraft, sulfite, or soda
pulp mill.
3. The process according to claim 1, wherein the cellulose pulp is
activated prior to step e) or step i) in order to increase
accessibility of cellulose dissolving chemicals.
4. The process according to claim 1, wherein an alkali metal
compound is sodium.
5. The process according to claim 1, wherein an alkali earth metal
compound is magnesium.
6. The process according to claim 1, wherein the hemicelluloses are
separated from the feed lignocellulosic material prior to cooking
in step b).
7. The process according to claim 1, wherein pentoses are separated
from cellulose pulp after cooking step b) or step h) but prior to a
cellulose dissolving step.
8. The process according to claim 30, wherein xylane is separated
from cellulose pulp by cold or warm extraction with an aqueous
extractant comprising sodium hydroxide followed by precipitation of
xylane from the extractant by dilution of the extractant with
water, an alcohol, or by acidulating the extractant with an
acid.
9. The process according to with claim 8, wherein xylane is
separated from cellulose pulp by cold or warm extraction with an
aqueous extractant comprising sodium hydroxide followed by
precipitation of xylane from the extractant by dilution of the
extractant in an alcohol which is a lower monohydric
C.sub.1-C.sub.4 alcohol.
10. The process according to claim 8, wherein precipitated xylane
is transformed into furfural using an acidic process comprising
acidulation and distillation or catalytic distillation.
11. The process according to claim 1, further comprising performing
a cellulose activation step comprising one or more of swelling
cellulose in alkali hydroxide, electron beam treatment of
cellulose, steam explosion treatment of cellulose, hydrothermal
treatment of cellulose, and enzymatic treatment of cellulose.
12. The process according to claim 11, wherein the hydrothermal
treatment of cellulose comprises treating the cellulose in an
aqueous solution optionally containing additives, at a temperature
of from 100 to 200.degree. C. for a time period of from 0.5 to 5
hours.
13. The process according to claim 11, wherein activation of
cellulose by steam explosion treatment is performed continuously or
in a batch reactor comprising treating cellulose with steam at a
pressure in the range of from 2 MPa to 6 MPa for a period of time
of from 5 to 500 seconds, thereafter the cellulose, and optionally
dissolved xylane, is abruptly discharged into a vessel at about
atmospheric pressure.
14. The process according to claim 1, wherein spent cellulose
dissolving or cellulose shaping chemicals comprising alkali metal
compounds in step f) are directly or indirectly charged to a
causticising plant in step d) wherein alkali carbonate is converted
to alkali hydroxide.
15. The process according to claim 1, wherein spent cellulose
dissolving or cellulose shaping chemicals comprising alkali or
alkali earth metal compounds in step f) are directly or indirectly
recycled to a recovery boiler or a gas generator for recovery of
fresh alkali or alkali earth metal compounds.
16. The process according to claim 1, wherein spent cellulose
dissolving or cellulose shaping chemicals in step f) are directly
or indirectly recycled to a recovery boiler or a gas generator for
recovery of sulfur compounds.
17. The process according to claim 1, wherein spent cellulose
shaping chemicals in step f) are discharged from a cellulose
coagulation step.
18. The process according to claim 1, wherein spent cellulose
dissolving or cellulose shaping chemicals comprise one or more of
alkali hydroxide, alkali carbonate, alkali sulfate, alkali sulfite,
zinc compounds, and phosphates.
19. The process according to claim 1, wherein spent cellulose
dissolving or cellulose shaping chemicals in step f) are recovered
from a shaped cellulose washing stage.
20. The process according to claim 1, wherein one or more additives
are present to support dissolving or shaping of cellulose in step
e).
21. The process according to claim 20, wherein the one or more
additives is an amphiphilic compound.
22. The process according to claim 1, wherein the sulfur compound
of step e) is one or more of concentrated sulfuric acid, methane
sulfonic acid, ethane sulfonic acid, and aryl sulfonic acid, and
the phosphorous compound of step e) is concentrated phosphoric
acid.
23. The process according to claim 1, wherein the molten salt of
step e) consists of one or more of zinc chloride and lithium
salts.
24. The process according to claim 1, wherein sulfuric acid is
produced from reduced sulfur compounds originating in a recovery
boiler or gas generator wherein at least a portion of the sulfuric
acid is used in a cellulose dissolving or cellulose shaping step or
for manufacturing of phosphoric acid.
25. The process according to claim 24, wherein the reduced sulfur
compounds substantially in the form of hydrogen sulfide are
liberated from green liquor formed by dissolving a smelt from a
recovery boiler or separated from a gas generator gas wherein the
hydrogen sulfide is oxidized to sulfur oxides and dissolved in
water to form an acidic solution.
26. The process according to claim 25, wherein an acidic solution
is used in a cellulose shaping or coagulation step and/or for
precipitating lignin from a spent cooking liquor.
27. The process according to claim 1, wherein substantially no
sulfur compounds are present during cooking in the cooking step
b).
28. The process according to claim 1, wherein substantially sulfur
free lignin is recovered from spent cooking liquor prior to
charging cooking liquor to a recovery boiler or gas generator.
29. The process according to claim 1, wherein the cellulose pulp of
step c) has a lignin content below about 1%.
30. The process according to claim 7, wherein the pentoses comprise
xylanes.
31. The process according to claim 21, wherein the amphiphilic
compound is selected from the group consisting of an ionic
surfactant, a non-ionic surfactant, a polyethylene glycol compound,
urea, thiourea, lecithin, betaine, and guanidine.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a process for the
manufacturing of shaped cellulose materials from lignocellulose. A
dissolving grade pulp rich in alfa cellulose is manufactured and
dissolved in an aqueous alkaline or acidic solvent system forming a
solution suitable for shaping new cellulose structures including
fibers, films and cellulose derivatives. More particularly the
present invention is directed to a process for combining the
production of shaped cellulosic material in a kraft, sulfite or
soda pulp mill wherein at least a part of the spent cellulose
dissolving or cellulose shaping chemicals are recovered in one or
more unit operations in the pulp mill chemical recovery cycle.
BACKGROUND TO THE INVENTION
[0002] Current industrial processes for pulping wood and other
sources of lignocellulosic material such as annual plants, and
processes for bleaching the resultant pulp have evolved slowly over
many decades. To remain competitive, the mature pulp and paper
industry is seeking new markets for the products produced in pulp
mills. It is of particular interest to further refine cellulose and
to valorize hemicelluloses and lignin.
[0003] Dissolving pulp is a low yield (30-40% by weight on wood)
bleached chemical wood pulp that has high alfa cellulose content
(>90%). This pulp has special properties, such as a high level
of brightness and high purity. Dissolving pulp is used for
production of regenerated cellulose products. The dominant process
for manufacturing regenerated cellulose fibers, the viscose
process, is suffering from a high environmental burden and high
energy demand. The viscose process is using large quantities of
carbon disulfide a chemical often contaminated with foul-smelling
impurities, such as carbonyl sulfide, hydrogen sulfide and organic
sulfides. Even the best of current technology is unable to suppress
the odors emitted in viscose plants. Furthermore there is no
efficient chemicals recovery process for recovery of spent
dissolving and coagulation chemicals.
[0004] The NMMO (N-Methylmorpholine oxide) process, a rather new
non-derivatizing process for producing regenerated cellulose fibers
is emerging as an alternative to the viscose process, however,
recovery of the NMMO solvent is complicated, energy demanding and
costly. The Chinese patent application CN 101280476 is directed to
a new method for recycling of NMMO solvent using cationic and
anionic resins.
[0005] The European patent application EP 1900860 is directed to a
process for dissolving cellulose in a sodium hydroxide urea
mixture. While this process may have an advantage in comparison to
the viscose and NMMO processes there is no suggestion on how to
recover the dissolving/coagulation chemicals.
[0006] Dissolving types of pulps, whether they are produced by a
prehydrolysis kraft process or sulfite process, are traditionally
used for manufacturing of viscose or cellulose derivatives such as
cellulose esters, rayon fibers and cellophane. Rayon is a soft
textile material, used in mostly tops, coats and jackets. Viscose
material can be produced either from dissolving grade pulp or from
cotton linter fibers. The manufacturing process starts by treating
the fibers with sodium hydroxide (mercerization). The mercerized
pulp is thereafter mixed with carbon disulfide to form cellulose
xanthate, a cellulose ester. The cellulose xanthogenate is
dissolved in sodium hydroxide forming a viscous cellulose solution.
The cellulose solution or viscose is extruded into an acidic bath
either through a slit to make cellophane, or through a spinneret to
make rayon. In the acidic environment the xanthogenate ester is
decomposed into cellulose and sulphurous compounds. A portion of
the carbon disulfide is recovered and recycled to treat new
cellulose.
[0007] European Patent EP1521873 is directed toward a process for
the manufacture of solid regenerated viscose fiber describing
certain new features of the traditional viscose process.
[0008] The viscose process was developed well over hundred years
ago and the process still has a dominant position on the market for
production of regenerated cellulose. For more details on the
viscose process (and NMMO process) reference is made to
"Regenerated Cellulose Fibres" The Textile Institute, Ed. Calvin
Woodings, Cambridge 2001.(ISBN 1 85573459 1)
[0009] Dissolving pulp can be manufactured by alkaline (kraft,
soda) and acidic (sulfite, bisulfite) pulping processes. In the
kraft process the cooking liquor is made up of sodium hydroxide and
sodium sulfide, in a nonsulfur soda pulp mill the sodium sulfide
is, in some locations, at least partly, replaced with
anthraquinone. Sulfite mills uses sodium sulfite or magnesium
sulfite/bisulfite as the active cooking chemicals. The chemicals
recovery cycle in a pulp mill include a recovery boiler,
evaporation plant, sulfur dioxide recovery units (for sulfite
mills) and recausticising plant (alkaline pulp mills). For a
detailed description of kraft, soda and sulfite chemical pulping
reference is made to "Chemical Pulping" Book 6A, Ed. Johan
Gullichsen, 2000. (ISBN 952-5216-06-3) and "Pulp and Paper
Manufacture" Volume 4. Sulfite Science & Technology" Ed. by O.
V. Ingruber, 1985. (ISBN 1-919893-22-8).
[0010] When the objective of the cellulose pulping operations is to
produce a dissolving type of pulp the target physical quality
parameters for the product are different than target quality
parameters for paper pulp. Tear and tensile strength is no longer
important while cellulose pulp purity is of essence (low lignin
content, low metals and ash content.
[0011] It is apparent that there is a need for a new and more
efficient cellulose dissolving process to replace the traditional
viscose process.
SUMMARY OF THE INVENTION
[0012] The present invention is directed to a process for combining
the production of cellulosic fiber products in a kraft, sulfite or
soda AQ pulp mill with a process for dissolving cellulose using a
new solvent system wherein at least a part of the spent cellulose
solvent chemicals are recovered in one or more unit operations in
the pulp mill chemical recovery cycle.
[0013] One objective of the present invention is to establish an
efficient cellulose dissolving process for making shaped cellulosic
material integrated into a cellulose pulp mill.
[0014] Another objective of the present invention is to provide a
low capital intensity and environmentally superior process for the
manufacturing of a cellulose dope suitable for shaping into new
cellulose fibers, films or cellulose derivatives.
[0015] The objectives of the present invention are accomplished by
using alkaline or acidic cellulose solvents for dissolving
cellulose said solvents optionally comprising an amphiphilic
additive with a capacity for breaking up the crystallinity of
cellulose. At least part of the solvent chemicals used for
cellulose dissolving or cellulose shaping are recovered in the same
process equipment, together with recovery of delignification
chemicals, oxygen delignification chemicals or pulp bleaching
chemicals. Recovered cellulose solvent and chemicals are recycled
from chemicals recovery to dissolve cellulose or shape cellulose to
new fibers, films or cellulose derivatives.
DETAILED DESCRIPTION OF THE INVENTION
[0016] The process of the present invention thus relates to a
dissolving cellulose pulp manufacturing and cellulose dissolving
process with an integrated recovery system for recovery of pulping
chemicals and recovery of chemicals for dissolving or shaping
cellulose. The subject process is carried out on in several steps
wherein the first step involves physical and chemical treatment of
lignocellulosic material such as wood or annual plant material in
order to increase accessibility of the lignocellulosic
material.
[0017] Following the chemical and physical pretreatment the
material is cooked in an alkaline or acidic buffer solution
optionally in the presence of one or more active chemical reagents
in order to obtain a delignified brown stock cellulosic pulp. The
brown stock pulp is optionally further delignified by oxygen.
Cellulose rich pulp is thereafter bleached using environmentally
friendly chemicals such as ozone, chlorine dioxide and hydrogen
peroxide in order to obtain a final dissolving pulp product with
low lignin content and desirable physical properties. The spent
cellulose liquor generated in the process comprising lignin
components and spent chemical reagents is concentrated by
evaporation followed by full or partial oxidation in a gas
generator. In the gas generator a stream of hot raw gas and a
stream of alkaline chemicals and chemical reagents are formed. The
alkaline chemicals are discharged from the gas generator and
further treated for subsequent recycle and reuse in the pulp
manufacturing process. Alternatively or combined with partial or
full oxidation the spent liquor is treated with an acid for
recovery of lignin.
[0018] The dissolving pulp produced by the above referenced
procedure is dissolved in a solvent to form a substantially
homogeneous cellulose rich solution or gel. The pulp dissolving
step is advantageously performed directly adjacent to a dissolving
pulp plant.
[0019] In the following section the invention is described in more
detail starting with the feed material preparation.
i) Feed Material Preparation and Hexose Hemicelluloses removal
[0020] Both hardwoods such as eucalyptus, acacia, beech, birch and
mixed tropical hardwood and softwoods such as pine, spruce and
hemlock can be used for manufacturing of a dissolving pulp suitable
for dissolving in the cellulose solvent system of the present
invention.
[0021] Hemicelluloses have, at least partly, to be removed from a
dissolving grade pulp. Hemicelluloses removal can be performed by
an acidic or alkaline prehydrolysis extraction procedure prior to
cooking or, in the case of pentoses such as xylane, by extraction
from the pulp product after cooking. Depending on the application,
the hemicelluloses content in the final dissolving pulp product
should be below about 7% by weight and preferably below about 3% by
weight.
[0022] In order to remove hemicelluloses from the lignicellulosic
feed material the cooking step may be preceded by a chips
prehydrolysis step. Such a treatment would, in addition to removal
of hemicelluloses, increase the accessibility of cooking chemicals
to the interior of the wood structure and decrease the effective
alkali requirement in subsequent pulping operations. In particular
a prehydrolysis stage will be applied when the feedstock to the
process of the present invention contains a substantial amount of
hexoses such as glucomannans.
[0023] A variant of prehydrolysis in this context is autohydrolysis
which essentially is a steam hydrolysis of the lignocellulosic
material at temperatures of 175-225.degree. C..
[0024] Under autohydrolysis conditions, the hemicellulose
components, as in prehydrolysis, are solubilized and the lignin is
partially hydrolyzed by cleavage of phenolic and ether
linkages.
[0025] In yet another variant of prehydrolysis, steam explosion
hydrolysis, the wood material is treated with steam at a
temperature of 200-250.degree. C. for a couple of minutes. This
treatment is followed by an explosively rapid discharge to
disintegrate the cellulosic substrate. In this type of process,
however, both chemical and mechanical attacks on the cellulosic
material leads to extensive depolymerization of the
carbohydrates.
[0026] The spent liquor resulting from the prehydrolysis treatment
should preferably be removed from the cellulosic material before
the pulp is subjected to further treatment. The spent liquor can be
removed through extraction strainers by washing or by pressing the
cellulosic material. After optional recycling the spent liquor is
discharged from the pretreatment step.
[0027] The pH during a prehydrolysis step can, depending on the
desired degree of hemicelluloses removal, be adjusted (by
temperature, time and additives) to any suitable value in the range
between about 0.5 to 7.0 preferably to a level between 1.0 and 5.0
and in the case of autohydrolysis a pH in the range of 4 to 6.
ii) Cooking/Delignification
[0028] After the cellulosic material has been subjected to any
pretreatment such as chipping, steaming or prehydrolysis; the
material is cooked in the presence of alkaline or acidic cooking
liquor based on a soluble alkali metal or alkali earth metal
compound. The base alkali metal is sodium and the base alkaline
earth metal is magnesium or calcium.
[0029] The objective of the cooking step is to separate cellulose
and lignin by dissolving the lignin in the cooking liquor. In case
of a kraft or soda process, alkaline cooking liquor consists
primarily of an alkali metal hydroxide or carbonate (and sodium
hydrosulfide in kraft). Acidic cooking liquor could be any sulfite
cooking liquor with a capacity to sulfonate and dissolve lignin. In
alkaline pulping processes alkali metal phosphates and alkali metal
boron compounds can be used as cooking liquor alkali.
[0030] The alkaline cooking liquor originate in the chemicals
recovery system of the pulping plant from where it is recycled,
with or without partial causticizing, to the cooking stage. When
boron based alkali is present in the in the cooking liquor the
causticising and lime reburning demand is lowered as boron
chemicals are partly autocausticised in the recovery boiler.
[0031] In a sodium sulfite or sodium bisulfite pulp mill the sulfur
needs to be separated from the sodium base in order to regenerate
the cooking acid. Sodium sulfate, sulfite and lignosulfonates
present in the sulfite process spent liquor (red liquor) are
forming a smelt comprising sodium sulfide in the recovery boiler.
The smelt is dissolved and sodium and sulfur compounds are
separated to form fresh cooking acid. In magnesium based pulp mills
the magnesium and sulfur are separated in the recovery boiler.
Solid magnesium oxide particles are removed and dissolved forming a
magnesium hydroxide solution. Gaseous sulfur oxides are scrubbed
with magnesium hydroxide forming fresh magnefite cooking
liquor.
[0032] Regardless of pulping process the temperature in the cooking
stage is maintained within the range from about 110.degree. C.to
about 200.degree. C. preferably from about 120 to 160.degree. C..
At the higher cooking temperatures, a shorter retention time in the
reaction vessel is required. A retention time of 30 to about 60
minutes can suffice at a temperature in the range of 170 to
200.degree. C., while from 60 to 360 minutes may be necessary to
obtain the desired result at cooking temperatures lower than about
170.degree. C..
[0033] Traditional types of single or dual vessel continuous
digesters of the hydraulic or steam liquor phase type as well as
batch digesters where the wood material is retained in the reaction
vessel throughout the cooking procedure may be employed to contain
the cooking reactions.
[0034] The recovery of spent liquors from these steps can be
integrated in a known manner with the recovery of spent liquors
from an oxygen delignificaton stage. The spent liquors can be
concentrated by evaporation and combusted in a separate combustor
or gasifier or mixed with other spent liquors for further
treatment.
[0035] Delignification catalysts and other additives can be added
to the cooking stage of the present process. Some of these
additives are commonly used to increase the rate of delignification
during alkaline digestion of cellulosic materials.
[0036] Sulfur chemicals free pulping is of particular interest both
for environmental reasons but also for the possibility of sulfur
free lignin recovery. In a sulfur free alkaline pulping process
specific polyaromatic organic compounds can be added to be present
in the cooking stage, such compounds including anthraquinone and
its derivatives such as 1-methylanthraquinone,
2-methylanthraquinone, 2-ethylanthraquinone,
2-methoxyanthraquinone, 2,3-dimethylanthraquinone and
2,7-dimethylanthraquinone. Other additives with a potential
beneficial function in this stage include carbohydrate protectors
and radical scavengers. Such compounds include various amines such
as triethanolamine and ethylenediamine and alcohols such as
methanol, ethanol, n-propanol, isobutyl alcohol, neopentyl alcohol
and resorcinol and pyrogallol.
[0037] Anthraquinone and its derivatives constitute the preferred
organic additives for use in the cooking stage in a sulfur free
pulping configuration. The anthraquinone additives are preferably
used in quantities not exceeding 1% of the weight of the dry
cellulosic substances and more preferably below about 0.5%.
[0038] The optimum operating conditions and chemical charges in the
cooking stage of the process depend on several parameters including
the source and origin of the cellulosic raw material, the end use
of the product etc. These specific conditions can be readily
determined by the artisan for each individual case.
iii) Oxygen Delignification
[0039] After cooking/delignification the cellulosic material is
optionally subjected to a mechanical defibration treatment in order
to liberate the fibers, facilitating efficient contact between the
reactants in a following oxygen delignification stage. Defibration
can be achieved, in its broadest sense, by introducing a fibrous
accumulated material into a treatment apparatus in which the fibers
are, at least partially, loosened from each other by breaking the
chemical bonds between individual fibers and by leaving the bonds
affected by physical forces essentially undisturbed. Further
defibration of the treated fiber accumulations may be performed by
subjecting the material to shear forces of sufficient strength to
substantially and completely separate said fibers without cleaving
or dividing the solid, chemically bonded particles within the fiber
accumulations.
[0040] Oxygen delignification and bleaching with oxygen-based
molecules have become standard in conjunction with the
manufacturing of bleached kraft and sulfite pulp and the cost of
oxygen chemicals has come down significantly.
[0041] In analogy with an alkaline cooking step alkaline liquor is
also present during oxygen delignification. The alkaline liquor
comprises alkali metal hydroxide and carbonate. Other buffering
chemicals can be employed such as alkali metal phosphates and
alkali metal boron compounds. The alkaline liquor used in the
oxygen delignification stage in a kraft mill originates in the
chemicals recovery system more particularly from a causticising
unit where the alkalinity of the liquor is restored to a pH value
above about 13 (white liquor). In order to eliminate reduced sulfur
compounds in the alkaline liquor solution (white liquor), the
liquor can advantageously be oxidized (white liquor oxidation)
using air, oxygen and or/ozone.
[0042] The oxygen added to the oxygen delignification stage can
either be pure oxygen or an oxygen containing gas, the selection
based on oxygen cost and partial pressure needed in the reactor.
The total pressure in the reactor is made up of the partial
pressure of steam, oxygen and other gases injected or evolved as a
result of the reactions in the oxygen delignification process. The
partial pressure of oxygen should be kept in the range of from 0.1
to 2.5 MPa.
[0043] The key objective of the cooking step ii) and oxygen
delignification step iii) is to liberate and dissolve lignin from
the lignocellulosic material. If the cooking step ii) is operated
at harsh conditions and lignin content in the cellulosic material
is lower than about 2% after cooking, the oxygen delignification
stage may be omitted. The drawback of a configuration without
oxygen delignification is that the following step iv) will have to
be optimized also for lignin removal.
iv) Bleaching and Xylane Recovery
[0044] The cellulose pulp produced in accordance with the
procedures described herein is finally treated to obtain a high
quality dissolving pulp by alkali extraction and/or bleaching using
effective bleaching agents, such as chlorine dioxide, hypochlorite,
peroxide and/or oxygen, ozone, cyanamide, peroxyacids, nitrogen
oxides or combinations of any such bleaching agents, in one or more
steps.
[0045] Apart from bleaching to high brightness (over 88 ISO) the
pulp the lignin content is also lowered during pulp bleaching
operations. The lignin content of the cellulose pulp after
bleaching should be lower than about 2% preferably lower than about
0.5% by weight.
[0046] Apart from makeup alkali charged counter currently with the
pulp flow, a portion or all the alkali used in bleaching and
extraction stages is recycled from a pulp mill recausticising
plant. Any reduced sulfur in recycled alkali is removed by
oxidation.
[0047] The alkaline bleach plant filtrates are preferably recycled
counter currently back to an oxygen delignification stage. Acidic
bleach plant filtrates, specifically those originating from
chlorine dioxide, ozone, nitrogen oxide or other acidic treatment
stages, can be recycled directly or indirectly to a prehydroloysis
feed pretreatment stage.
[0048] When the feed material to the pulping process is a pentose
rich hardwood and partial removal of pentoses from the pulp product
is desired, pentoses such as xylanes can advantageously be removed
from the cellulose pulp in any position after the
cooking/delignification step prior to cellulose dissolving. The
extraction is performed by an aqueous extractant preferably an
aqueous alkaline metal hydroxide (cold extraction or hot
extraction) at conditions chosen for selectively dissolving
xylanes. Dissolved xylane is precipitated from the extractant by
acidulation with an acid such as carbon dioxide or by dilution of
the extractant with water or an organic solvent such as a
monohydric C1-C4 alcohol such as ethanol or iso-propanol.
Precipitated xylane is removed and the extractant is recycled, and
recausticised if needed, to treat new pulp.
[0049] Recovered pentoses (exemplified hereinabove with xylane) can
be exported from the pulp mill or upgraded on site to green
chemicals and polymers such as furfural, lactic acid, PLA etc.
Alternatively, recovered pentoses can be transformed on site into
furfural using an acidic process comprising an acidulation followed
by fractionation/distillation or transformed directly to furfural
in a catalytic distillation unit.
v) Chemicals and Energy Recovery
[0050] Spent pulping liquor is, with or without prior extraction of
lignin and other organic material, withdrawn to be further
processed in a recovery process to recover alkali or alkali earth
metal compounds and energy values.
[0051] The spent cooking liquor contains almost all of the
inorganic cooking chemicals along with lignin and other organic
matter separated from the lignocellulosic material. The initial
concentration of weak spent liquor discharged from the digester is
about 15% dry solids in an aqueous solution. Weak spent liquor is
concentrated to firing conditions in evaporators and concentrators
to a solids content ranging from about 65% to about 85%.
[0052] While a standard recovery boiler may be used for processing
spent pulping liquor, a chemicals recovery system based on
gasification or partial oxidation of the cellulose spent liquors in
a gas generator can also be used.
[0053] Gasification of carbonaceous material for the recovery of
energy and chemicals is a well established technology and emerging
as an alternative for recovery of chemicals and energy in pulp
mills. Cellulose spent cooking liquors contains a large fraction of
salty inorganic compounds with a low melting and agglomeration
point and although various fluidized bed concepts have been
disclosed for conversion of cellulose spent liquors, it is
generally agreed that a suspension or entrained flow gasifier is
more suitable for gasifiying spent cooking liquors.
[0054] Several types of gasifiers or gas generators can be used,
with minor modifications, in the practice of the present invention
including, for example, the gasifiers described in U.S. Pat. No.
4,917,763, U.S. Pat. No. 4,808,264 and U.S. Pat. No. 4,692,209.
These gasification systems are suitable for chemicals and energy
recovery from high sulfidity cellulose spent kraft and sodium base
sulfite cooking liquors. The sulfur chemicals are recovered as
alkali sulfides but a substantial portion of the sulfur will also
follow the raw fuel gas as hydrogen sulfide and carbonyl sulfide.
Entrained molten alkaline chemicals in the raw fuel gas are
separated from the gas stream in a cooling and quenching stage and
dissolved in an aqueous solution. The alkaline solution, called
green liquor, is causticized with lime to obtain a high alkalinity
white liquor, the traditional cooking chemical used in kraft
pulping operations.
[0055] On the other hand two stage reaction zone up draft gasifiers
designed for gasification of heavy hydrocarbons and coal can also,
with minor modifications, be used in the practice of the present
invention, such gasifiers described in e.g. U.S. Pat. No. 4,872,886
and U.S. Pat. No. 4,060,397.
[0056] Another gasifier with a suitable design for use in the
present invention is disclosed in U.S. Pat. No. 4,969,931.
[0057] Regardless of the type and design of gasifier or recovery
boiler, inorganic molten droplets and aerosols formed in the unit
is separated from the gas flow and dissolved in an aqueous
solution. The solution comprises alkaline compounds in a form
suitable, optionally via caustizicing, for use as alkali in oxygen
delignification, alkaline cooking stages and cellulose
dissolving/cellulose shaping stages of the present invention.
Causticizing is a well known unit operation in the art of alkaline
chemical pulping and is not described herein.
[0058] Are sulfur compounds present in the spent cooking liquor
these compounds will form alkali sulfides and alkali sulfate
depending on the design of the recovery unit. Alkali sulfide as
such is an effective pulping chemical/catalyst in the kraft
process. In sodium base sulfite mills the sodium and sulfur needs
to be separated to restore fresh cooking acid. Chemicals recovery
design and operation for kraft and sulfite mills is well known to
the artisan.
[0059] In addition to the chemicals recovered for pulping and
bleaching operations the recovery system of the pulp mill is also
used for recovery and restoration of cellulose dissolving,
cellulose coagulation and shaped cellulose washing liquids. These
aqueous liquids comprise alkali metal value and need to be restored
into active dissolving chemicals. Restoration of alkali metal
hydroxide for use in a cellulose dissolving stage may comprise one
or more of concentration in an evaporation plant, partial or full
oxidation combined with spent pulping liquor in a recovery boiler
or gas generator, causticizing in a recausticsing plant, and
treatment with oxygen in a white liquor oxidation plant. If
sulfuric acid or sulfurous acids are used in a cellulose
coagulation step such acids may be recovered from splitting sodium
and sulfur (by gasification and/or by acidulation of raw green
liquor) over oxidation of reduced sulfur compounds to sulfur oxides
which, when dissolved in water, form acidic liquors suitable for
use in a cellulose coagulation step.
[0060] Alkaline liquors produced in the recovery system of the
present invention can be subjected to an oxidative treatment with
an oxygen containing gas in order to eliminate any traces of
sulfide before the liquor is recycled and charged to the desired
cellulose dissolving/shaping, bleaching or oxygen delignification
stage of the present invention.
vi) Sulfur Free Lignin Recovery
[0061] In one embodiment of the present invention the pulping
process is substantially sulfur chemicals free. In this
configuration a portion of the lignin can advantageously be
extracted and separated from a spent liquor stream or digester
circulation stream prior to final concentration and discharge to a
recovery boiler or gasifier. A substantially sulfur chemicals free
lignin can be recovered in accordance with state of the art lignin
recovery technologies. The sulfur free lignin can be used as a raw
material or precursor for fine chemicals, carbon fibers, phenols,
and engineering plastic products or be used as a sulfur free
biofuel. Lignin can be precipitated from cellulose spent liquors
with solids content in the range of 3-30% supported by the action
of one or more acids including sulfur oxide acidic liquors and
carbon dioxide. Sulfur oxide acidic liquors can be produced on site
as described herein. Carbon dioxide can be recovered from gaseous
streams in the dissolving pulp mill. The total sulfur content of
washed lignin recovered by the procedure described herein
(including covalently bonded sulfur) is lower than about 1% by
weight of dry lignin, preferably lower than 0.5% sulfur and most
preferred lower than about 0.1% sulfur by weight.
vii) Dissolving of Pulp in Alkaline or Acidic Solvents and
Cellulose Shaping
[0062] A dissolving grade pulp imported to or produced in an
chemical pulp mill is dissolved in a cellulose solvent forming a
substantially homogeneous cellulose solution. The dissolved
cellulose is shaped into new fibers, films or cellulose derivatives
in one or more processing steps following the cellulose dissolving.
Cellulose shaping into new fibers can be performed by injection of
the cellulose solution through nozzles, directly or by air jet,
into a coagulation bath comprising coagulation chemicals. The
coagulation chemicals are characterized in that they are poor
cellulose solvents. Cellulose fibers are reformed (and drawn) in
the coagulation step into a filament or tow of regenerated
cellulose. The filament or tow can advantageously be further
converted to a cellulosic staple fiber for export from a cellulose
dissolving plant. Cellulosic staple fibers can be used in wide
range of end products such as textiles and hygienic consumer
products. Cellulose shaping can also performed by injection of the
cellulose solution onto a moving bed forming a nonwoven cellulose
fiber network (spunlaid nonwoven). Coagulation liquid may be added
to the moving bed, or thereafter, to strengthen the fiber network.
Various forms of hydro entanglement may be applied to obtain the
desired cellulosic network structure and strength. Cellulose
shaping may also be performed by reacting the dissolved cellulose
in homogeneous phase with a reactant forming cellulose derivatives
such as, for example, cellulose esters.
[0063] We have discovered that there is a great technical,
environmental and commercial advantage if the recovery of cellulose
dissolving and/or coagulation chemicals can be recovered in
conjunction with the recovery of cooking and oxygen delignification
chemicals. Surprisingly, the chemicals used in dissolving or
coagulation steps of the present invention may, provided the
selected conditions and additives are used, be recovered in
recovery systems used also for cooking and/or oxygen
delignification.
[0064] In order to obtain a high cellulose concentration in the
cellulose dope the dissolving pulp (obtained after cooking,
bleaching and hemicelluloses extraction) can be activated prior to
dissolving, primarily to increase accessibility for cellulose
dissolving chemicals. Activation may also partially decrystallize
the cellulose and shorten the cellulose molecules from a typical Dp
(degree of polymerization of anhydroglucose repeating units) in the
range of 700-1300 in the dissolving pulp to a range of 200-700. The
dissolving pulp activation can be performed by either of, or a
combination of, swelling in alkali hydroxide, enzymatic treatment,
electron beam treatment, hydrothermal treatment and steam explosion
treatment.
[0065] A hydrothermal treatment of the cellulose material may be
performed in a closed vessel (batch digester) at a temperature of
about 100 to 200.degree. C.for 30 minutes to 5 hours with or
without the presence of additives (such as weak organic acids).
Microwaves can be used for supply of energy to the dissolving pulp
activation stage.
[0066] Steam explosion activation may be performed continuously or
in batch by treating cellulose pulp or pulp slurry with steam at a
pressure in the range of 2 MPa to 6 MPa during 5 to 500 seconds.
Optionally the pH in the pulp slurry may be adjusted to a pH below
about 7 by the addition of alkali. The pulp is after steam
treatment abruptly discharged into a vessel at considerably lower
or atmospheric pressure. If a xylane rich pulp is treated by the
steam explosion activation treatment disclosed herein, the xylane
is to a considerably degree dissolved in the pulp slurry. Such
xylane can be recovered and upgraded in accordance with procedures
described herein.
[0067] Activated cellulose pulp is after any activation procedure
described herein transferred to a cellulose dissolving step
[0068] The most preferred cellulose dissolving chemical system is
based on alkali hydroxide in the presence of an amphiphilic
additive compound. Amphiphilic compounds are characterized in that
they possess both a hydrophilic and a lipophilic moiety. The alkali
hydroxide is preferably sodium hydroxide produced by causticising
sodium carbonate rich liquor. Sodium hydroxide may be pretreated
with oxygen or ozone in order to oxidize reduced sulfur. The
concentration of alkali hydroxide in the cellulose dope is below
20% by weight, preferably adjusted to around 10% by weight.
[0069] The amphiphilic compound is a polyelectrolyte, surfactant
(anionic, cationic, nonionic or zwitterionic),polyethyleneglycol,
urea, thiourea, or guanidine. Specifically preferred amphiphilic
compounds include SDS (sodiumdodecyl sulfate), polyethylene
glycol/polypropylene glycol copolymers and lecithin. The additive
is used in a concentration below about 10% by weight of the
cellulose in the solution, preferably below about 3%.
[0070] The dissolving of cellulose is preferably performed in
temperature range of minus 15.degree. C. to plus 20.degree.C..
[0071] Provided the cellulose spinning system can accommodate
highly viscous dopes, the concentration of cellulose in the
cellulose dope can be as high as 25% by weight. The cellulose
polymers may be ordered in the dope in the form of a liquid
crystalline phase, such phase having advantageous properties for
cellulose shaping into new fibers with high tenacity.
[0072] The concentration of cellulose can also be kept lower (5-15%
by weight) in order to have a low viscosity and suitable cellulose
dope rheology for performing homogeneous reactions or spinning in
conventional spinning machinery.
[0073] The spinning dope comprising cellulose can be injected into
a coagulation liquid bath through fine nozzles to form a cellulose
filament or tow; alternatively the spinning dope can be injected
into a moving bed forming a nonwoven web. The coagulation liquid is
any suitable liquid with a low or very low capacity to dissolve
cellulose. Advantageously the coagulation liquid comprises an
alcohol such as a monohydric alcohol (ethanol, methanol, propanol,
iso-propanol, acetone or a polyhydric alcohol (glycerol). The
coagulation liquid may also be composed of acids (organic acids
such as acetic acid, formic acid and mineral acids such as sulfuric
acid) or a phosphate and/or sulfate salt. Furthermore the
coagulation liquid can be a diluted cellulose dissolving liquid.
Various types of additives including zink compounds may be added to
the coagulation liquid to promote formation of a cellulose fiber
with the desired physical properties and geometrical shape. Spent
coagulation liquids are at least partly recycled to and recovered
in the same equipment used for recovery of chemicals for cooking,
oxygen delignification or pulp bleaching.
[0074] Whether cellulosic films, nonwoven webs, hydroentanglement
bonded material, filaments or tows are produced the cellulosic
product is normally washed in one or several stages with washing
liquids. The system for recycle and recovery of washing liquids is
advantageously integrated in the pulp mill energy and chemicals
recovery system.
[0075] In addition to the alkaline cellulose solvent system
disclosed herein concentrated phosphoric acid, organosulphonic
acids (methane sulfonic, ethane sulfonic or aryl sulfonic) and/or
molten salt hydrates preferably comprising zink chloride and/or a
lithium anion such as LiClO.sub.4*3H.sub.2O can be used for
cellulose dissolving and cellulose shaping. Coagulation liquid in
this configuration can be water, acetone, alcohol, phosphate salts
or alkali hydroxide. While energy integration with a pulp mill is
straightforward with these dissolving chemicals, the regeneration
of acids and molten salt hydrate chemicals from the coagulation
liquids is considerably more complicated than the recovery of
alkaline cellulose solvents. Fresh phosphoric acid for cellulose
dissolving can be produced by treating phosphate salts recovered
from a coagulation bath with sulphuric acid (eventually generated
on site) forming sulphate and the desired phosphoric acid.
[0076] Sulfate salts in spent chemicals (by-product from producing
phosphoric acid on site or present in spent cellulose coagulation
liquid) can be charged directly or indirectly to a recovery boiler
or gas generator for reduction to sulfides. Sulfides can then in
turn be separated for example in the form of hydrogen sulfide gas
which gas is oxidized to sulfur oxides. Sulfur oxides produced in
this way can be dissolved in water forming sulphuric acid or an
acidic sulphur oxide solution suitable for use in a cellulose
coagulation step.
[0077] According to the present invention there is provided a
process for the production of a shaped cellulose material from
lignocellulose and the recovery of chemicals used in said process
as set forth in independent claim 1.
[0078] Further features and specific embodiments of the invention
are set forth in the dependent claims.
* * * * *